A fuel cell has been proposed as a clean, efficient and environmentally responsible power source for various applications. In particular, individual fuel cells can be stacked together in series to form a fuel cell stack capable of generating a quantity of electricity sufficient to power an electric vehicle.
One example of the fuel cell is a Proton Exchange Membrane (PEM) fuel cell. The PEM fuel cell includes a membrane-electrode-assembly (MEA) that generally has a thin, solid polymer membrane-electrolyte having an anode and a cathode with a catalyst on opposite faces of the membrane-electrolyte. The MEA is generally disposed between a pair of porous conductive materials, also known as gas diffusion media, which distribute gaseous reactants, e.g. hydrogen and oxygen/air, to the anode and cathode layers. Collectively, the MEA and the gas diffusion media are known as “softgoods”. The hydrogen reactant is introduced at the anode where it reacts electrochemically in the presence of the catalyst to produce electrons and protons. The electrons are conducted from the anode to the cathode through an electrical circuit disposed therebetween. Simultaneously, the protons pass through the electrolyte to the cathode where an oxidant, such as oxygen or air, reacts electrochemically in the presence of the electrolyte and catalyst to produce oxygen anions. The oxygen anions react with the protons to form water as a reaction product.
The softgoods of the PEM fuel cell are sandwiched between a pair of electrically-conductive bipolar plates which serve as current collectors for the anode and cathode layers. The bipolar plates include a plurality of lands and flow channels for distributing the gaseous reactants to the anodes and cathodes of the fuel cell. The bipolar plates serve as an electrical conductor between adjacent fuel cells and are further provided with a plurality of internal channels adapted to exchange heat with the fuel cell when a coolant flows therethrough. The typical bipolar plate is a joined assembly constructed from two separate unipolar plates. Each unipolar plate has an exterior surface having flow channels for the gaseous reactants and an interior surface with the internal coolant flow channels. In order to conduct electrical current between the anodes and cathodes of adjacent fuel cells in the fuel cell stack, the paired unipolar plates forming each bipolar plate assembly are mechanically and electrically joined.
A typical bipolar plate assembly design is known as a “nested” configuration. In the nested configuration, the channels and lands formed on the interior surfaces of the respective unipolar plates are aligned and mated. Nonlimiting examples of nested configurations are described in U.S. Pat. No. 6,974,648 and in U.S. Pat. App. Pub. No. 2006/0127706, the disclosures of which are incorporated herein by reference in their entireties. A known method of preparing a bipolar plate assembly having a nested configuration includes clamping a pair of unipolar plates with matching channeled regions around a perimeter of the unipolar plates. A pressure is applied to the perimeters of the unipolar plates, for example, with dead weight or clamps. The perimeter is welded to seal bipolar plate assembly.
The bipolar plate assembly is known to occasionally exhibit an undesirable variation in thickness across a surface area of the bipolar plate assembly. Thickness variation across the bipolar plate assembly generally results from an improper nesting of the channels and lands of the unipolar plates when assembled to form the bipolar plate assembly. The improper nesting may, in part, be due to differences in lateral springback of the unipolar plates, whereby one unipolar plate may have a greater amount of lateral springback due to dimensional differences following a forming thereof, for example, by a stamping operation. The thickness variation may manifest itself as a “trapped wave” of material within the nested regions of the bipolar plate assembly, particularly when any excess or slack material is present when the perimeters of the unipolar plates are constrained. The trapped wave of material may result in a gap or a separation between the unipolar plates.
Spot welding the unipolar plates together within the active area is one known approach to providing a low resistance to electron flow through the bipolar plate assembly. However, the abovementioned separation between the unipolar plates may present difficulties in spot welding within the active area of the bipolar plate assembly. Attempting to weld at a location where a separation exists causes a perforation in the unipolar plate. The perforation forms a leak path for fluids in the fuel cell stack. The separation between the unipolar plates is therefore undesirable.
Testing has shown that the final nesting of the channeled regions is substantially completed under the build loads associated with assembly of the fuel cell or fuel cell stack. Under these compressive forces, the channels and lands are eventually “seated” together. However, the employment of fuel cell compression to complete the nesting of the channeled regions results in undesirable localized stresses on the softgoods. Both variation in bipolar plate thickness and uneven distribution of softgood stresses is known to negatively affect fuel cell performance. Non-nested regions, in particular, may cause an undesirable level of compression under typical stack loads. The undesirable level of compression, combined with other factors such as temperature and humidity, may lead to electrical shorts through the PEM of the fuel cell.
There is a continuing need for a method and assembly fixture for reducing a variation in thickness of the bipolar plate assembly. Desirably, the method militates against an improper nesting of the unipolar plates during assembly of the bipolar plate.